1. Field of the Invention
The present invention relates to a semiconductor optical device favorable for a semiconductor laser and the like and a manufacturing method thereof.
2. Description of the Related Art
A Semi-Insulating Buried-Heterostructure (SIBH) is known as a current confinement structure in a semiconductor optical device such as a semiconductor laser. As another example of the current confinement structure, there is a structure, in which a mesa structure portion including an active layer is buried with a pnpn thyristor structure; however, the SIBH structure is more effective to reduce parasitic capacitance, which limits a modulation bandwidth. Accordingly, the SIBH is better to execute a modulation in a high bit-rate such as 10 Gb/s and 40 Gb/s.
In the SIBH structure, a mesa structure portion including an n-type InP clad layer, an active layer and a p-type InP clad layer is formed on an InP substrate having an n-type conductivity. The mesa structure portion is buried with a semi-insulating InP layer (burying layer). The semi-insulating InP layer used as the burying layer is an Fe-doped InP layer, and, since Fe generates a deep level, the resistance of the semi-insulating InP layer is high. In addition, Zn is used as a dopant of the p-type InP clad layer.
The SIBH structure has the above structure, but it is known that Zn in the p-type InP clad layer and Fe in the burying layer often cause an inter-diffusion. In other words, Zn diffuses from the p-type InP clad layer into the burying layer and Fe diffuses from the burying layer into the p-type InP clad layer. Such a phenomenon is described in, for example, non-Patent Document 1 (E. W. A. Young, et. al., “Zinc-stimulated outdiffusion of iron in InP”, Appl. Phys. Lett., 56, pp 146, 1990). Also, Patent Document 1 (Japanese Patent No. 3257045 (Japanese Patent Application Laid-Open No. Hei 6-37392)) discloses results of secondary ion mass spectrometry (SIMS) of Fe and Zn, regarding the above inter-diffusion.
When such an inter-diffusion of Fe and Zn occurs, an insulation performance of an area in the burying layer near its interface with the p-type InP clad layer is deteriorated and this causes an increase in a leakage current in burying layer. As a result, an efficiency of carrier injection to the active layer is decreased so that problems such as an increase in threshold current and deteriorations of optical output characteristics may occur. Further, in an electroabsorption modulator, a parasitic capacitance is increased and modulation bandwidth may be narrowed.
In view of these problems, Patent Document 1 describes a technique for doping not only Zn but also Fe to the p-type InP clad layer in the same concentration with that of the burying layer, in order to suppress the inter-diffusion. In other words, it describes a technique, in which the same Fe concentration of 6×1016 cm−3 is applied to both the p-type InP clad layer and the burying layer. Using this technique, since the Fe in the burying layer hardly diffuses into the p-type InP clad layer, the inter-diffusion can be suppressed. As a result, a high insulation performance of the burying layer is maintained and a leakage of current in the burying layer is suppressed so that a superior optical output characteristic can be obtained. According to the Patent Document 1, a desired object is attained as described above.
In contrast, a demand for a further lowered resistance in the semiconductor optical devices such as a semiconductor laser has been increased and it has been studied to increase the impurity concentration in the p-type InP clad layer. The Zn concentration (impurity concentration) in the p-type InP clad layer in Patent Document 1 is 5×1017 cm−3; however, recently, it is assumed that a preferable concentration is 1.0×1018 cm−3 or more.
However, according to results of an experiment conducted by the inventors of the present application, it is found that the inter-diffusion of Fe and Zn cannot be sufficiently suppressed when the Zn concentration in the p-type InP clad layer is made 1.0×1018 cm−3 or more with the technique described in Patent Document 1. The details will be described later.
Related art is described in Patent Document 2 (Japanese Patent Application Laid-Open No. 2003-114407 (paragraphs 0008-0009, FIG. 9)).